1. Field of the Invention
The present invention relates to thermal transfer recording media and print-recording methods therewith, in which the ink used is melted or sublimated by heating.
More particularly, the present invention relates to current-conducting thermosensitive recording media used for non-impact recording.
2. Description of Related Art
Various kinds of print-recording methods of recording picture information on ordinary paper are achieved through conversion of an electrical signal into thermal energy. Examples of these print-recording methods are (i) a thermal heat transfer method, (ii) a current-conducting transfer method, and (iii) a thermal transfer printing method.
In the (i) thermal heat transfer method, a thermal head used as a printing head is brought into contact with the back side of a thermally conductive recording medium (ink donor film) coated with low-melting ink. Thermal pulses are applied to the ink donor film corresponding to the picture information so that ink, at appropriate positions, is melted by thermal conduction. The melted ink is transferred onto ordinary paper (hereinafter referred to as "recording paper"). In the (ii) electrical transfer method, ink is melted by heat generated from needle electrodes, so that the melted ink is transferred to the recording paper. In the (iii) thermal transfer printing method, a thermogenetic resistor layer and a return electrode are provided on an ink support of moderate resistance. Needle electrodes are brought into contact with the ink support to form current paths in the medium which, in turn, melt the ink selectively. The melted ink is then transferred to the recording paper.
In the (i) thermal head transfer method, thermal pulses are transmitted to the ink layer through a base, such as condenser paper or the like, which forms the ink donor film. Because long-distance transmission depends on thermal conduction, the time required for printing is not less than 1 mS per picture element (dot). Accordingly, the printing speed is slow. Further, energy transmitted to the ink layer is reduced because loss occurs in the base paper portion. For this reason, no material except wax can be used as ink material, and thus there are few alternatives available in the selection of material. Furthermore, ink transfer control to recording paper is difficult because in practice, multistageous control of dot size for tone expression is troubling.
In the (ii) current-conducting transfer method, allowance for electrical conductivity to the ink layer makes color control, and therefore color recording, difficult. Further, electrical conduction loss is large and mechanical characteristic in the support portion is poor. Because printing dots are unstable and electrical anisotropy in the support portion is unsatisfactory, the method is disadvantaged in that energy loss in the portion is large.
In the (iii) thermal transfer printing method, the ink support does not have anisotropy and enlargement of dots occurs. Because leak current which does not relate to thermogenesis is large, energy efficiency is poor. Because the ink support must have some degree of resistance, the method has a problem in that contacting resistance between each electrode and the ink support necessarily increases.
To solve the aforementioned problems, a print-recording method using a non-impact type current-conducting thermosensitive recording ink medium (print ribbon) has been disclosed, for example, in Japanese Patent Unexamined Publication No. 56-93585.
FIG. 1 is a view for explaining the principle of the conventional print-recording method. In the method, a current-conducting thermosensitive recording ink medium 1 comprises an upper layer 2, a lower layer 3, a conductor layer 4 and an ink layer 5. A print electrode 6 and a ground electrode 7 are in contact with the low-resistance upper layer 2. The ground electrode 7 and the print electrode 6 are mounted such that they are not in contact with each other. When a voltage is applied to the print electrode 6 in response to a picture signal, a current flows into the upper layer 2, lower layer 3 through conductor layer 4, successively and in the lower layer 3 and upper layer 2 again and reaches the ground electrode 7. In this configuration, the upper layer 2 generates a small amount of heat but the lower layer 3 generates a large amount of heat in its portion of the current path. Thermal energy at the thermogenetic portion reaches the ink layer 5 through the conductor layer 4 to cause melting of ink. FIG. 2 shows a portion of a recording apparatus using the conventional current-conducting thermosensitive recording ink medium. Recording paper (ordinary paper) 8 is put on the ink layer 5 side of the current-conducting thermosensitive recording ink medium 1. A pressure roll 9 has the double function of urging the current-conducting thermosensitive recording ink medium 1 against the print electrode 6 and feeding the current-conducting thermosensitive recording ink medium 1 together with the recording paper 8 in a predetermined direction (in a direction perpendicular to the longitudinal direction of the print electrode).
When voltage is applied between the print electrode 6 and the ground electrode 7 in the recording apparatus, a current passes through the lower layer 3 at two positions. Accordingly, thermal energy is generated at two positions corresponding to one picture element. Ink at only one position corresponding to the print electrode 6 is transferred to the recording paper 8. In other words, ink at the other portion corresponding to the return electrode 7 is wastefully heated. Consequently, efficiency in use of energy is poor.
Further, in the current-conducting thermosensitive recording ink medium 1, electrical contact is made at the two places of the print electrode 6 and the ground electrode 7. Accordingly, contacting resistance increases resulting in wasted energy.
Having described a conventional current-conducting thermosensitive recording ink medium 1 where the ink medium 1 has an upper layer 2 and a lower layer 3, the conventional ink medium may be of another structure in which the two layers are combined to form a monolayer. However, the same problem exists in the ink medium having such a monolayer structure.